Microbiome derived acidity protects against microbial invasion in .

Microbial invasions underlie host-microbe interactions that result in microbial pathogenesis and probiotic colonization. While these processes are of broad interest, there are still gaps in our understanding of the barriers to entry and how some microbes overcome them. In this study, we explore the effects of the microbiome on invasions of foreign microbes in . We demonstrate that gut microbes and improve survival during invasion of a lethal gut pathogen and lead to a reduction in microbial burden. Using a novel multi-organism interactions assay, we report that inhibits the growth of three invasive Gram-negative bacteria, while prevents this inhibition. A series of and experiments revealed that inhibition by is linked to its ability to acidify both internal and external environments, including culture media, fly food, and the gut itself, while diminishes the inhibition by quenching acids. We propose that acid produced by the microbiome serves as an important gatekeeper to microbial invasions, as only microbes capable of tolerating acidic environments can colonize the host. The methods described herein will add to the growing breadth of tools to study microbe-microbe interactions in broad contexts.